1. Field of the Invention
The present invention relates to vehicle braking systems and more particularly to an improved electric power brake.
2. Description of the Related Art
Vehicle braking systems have evolved from simple mechanical brakes to hydraulic brakes and then to power assisted hydraulic brakes. One very common power brake mechanism employs a housing intermediate an operator brake actuating pedal mechanism and a master cylinder which, when enabled, supplies fluid pressure to individual wheel brake cylinders or actuators. The housing includes a piston or diaphragm normally exposed on both sides to vehicle manifold vacuum. When the operator actuates the brake pedal, atmospheric pressure is admitted to one side of the piston supplying additional force to the master cylinder piston and enhanced brake line pressure to the individual wheel cylinders. Operator applied pedal force (without boost) is transmitted to the master cylinder through the boost mechanism if no vacuum is available. Vacuum boosters of this type are illustrated in numerous patents, for example, U.S. Pat. No. 5,943,863.
Many vehicles, for example, hybrid vehicles or those employing diesel engines, lack a vacuum source and may utilize a power steering pump or a separate dedicated pump as a hydraulic pressure source, or may employ a separate air compressor for air brakes. All of these systems rely on operation of the vehicle engine to augment operator brake pedal input force.
Several patented systems eliminate the dependence on manifold vacuum or other power source dependent on engine operation by substituting an electromechanical mechanism for above described vacuum booster. In U.S. Pat. No. 4,395,883, for example, brake pedal force is amplified by an electric motor acting through a gear set which converts, by way of a ball screw arrangement, rotary motion and torque from the motor into linear motion and force to energize the master cylinder piston. Operator input push rod forces are sensed, amplified and modulated by electronic means. The electric motor disengages from the gear set at rest or if there is an electrical failure, so that there is a manual mode of operation available when necessary. A solenoid is used to operate a power clutch to separate the power and no-power apply modes. Suitable controls are provided by force sensing means such as a piezoelectric crystal or some other suitable pressure transducer, and suitable circuitry including control logic in the nature of a microprocessor.
U.S. Pat. Nos. 6,574,959 B2 and 6,758,041 B2 disclose similar systems. The 6,574,959 patented arrangement employs a transmission system including a belt wrapped around pulleys to transfer power from a motor to a ball screw, and suggests belts, toothed belts, chain belts or gears and that the pulleys can be sprockets or the like. In the 6,758,041 patent, the ball screw assembly is operatively connected by pinion and a ball screw drive gears between the output shaft of the electric motor and the booster shaft for converting torque from the motor into axial force applied to the booster shaft. Brake pedal input force is measured by a piezoelectric or magneto-restrictive sensor. To increase the speed at which the booster shaft retracts along the axis, and to ensure that the booster shaft moves to a fully retracted position, the booster includes a retraction spring in the form of a torsion or wrapped spring operatively connected between the ball screw drive gear and the housing.
Each of these patented electric booster arrangements is disposed intermediate the brake pedal push rod and the master cylinder input piston rod just as in the case of the traditional vacuum booster. Each employs a rather complex mechanical coupling between the motor and master cylinder piston rod. Electric brakes employing electromagnets at individual wheel brake locations to force braking surfaces together have also been suggested.
It is desirable to simplify complicated drive mechanisms, e.g., by eliminating complex driving transmissions, solenoid operated clutches and the like while retaining independence of engine operation and a vehicle braking capability despite any boost power source failure.